Seven engineers who work for the Nuclear Regulatory Commission have called on their employer to force nuclear power plant owners to fix a design flaw that could affect emergency core cooling systems. If the flaw is not fixed, they say, the plants should be shut down.
The engineers, filing as private citizens, have submitted a formal petition, the process used by the public when asking the federal agency to take action.
The design flaw in the system used to detect electrical supply problems has been known since an incident at Exelon’s Byron 2 Nuclear Station in Illinois in 2012. The plant went into automatic shutdown and remained so for a week after an undetected open-phase condition in its electrical system.
An open phase involves a damaged circuit that has failed or is providing imbalanced voltage to equipment.
“An open phase at a nuclear plant, if not detected and isolated by tripping breakers, could damage the motors of lots of emergency equipment,” explained David Lochbaum, director of the Nuclear Safety Program for the Union of Concerned Scientists. “If so, even when the bad power is turned off and good power from a source like the emergency diesel generators is provided, the broken motors could prevent the emergency equipment from running.”
If the motors burn out, the plant loses its backup ability to cool the reactor.
The design flaw in Byron’s detection system, which has since been fixed, exists in nearly all U.S. nuclear reactors, including Pilgrim Nuclear Power Station in Plymouth, although Pilgrim has never experienced an open-phase condition in its system.
In a letter send by Entergy Corp. to the NRC in 2013, Entergy experts stated that Pilgrim’s relay systems weren’t designed to detect an open single phase of the three-phase system, but the existing protective circuitry would separate the plant’s power lines from an off-site connection failure or sustained imbalance in voltage.
According to the petition, 13 open-phase events have occurred in the last 14 years in the United States and internationally.
The Nuclear Regulatory Commission required all plants in 2012 to provide information regarding their detection systems for open-phase conditions.
Plants have not yet been required to put in new systems to resolve the design flaw permanently. They have been given until the end of 2018 to permanently address the problem.
The NRC is satisfied with actions taken to date, according to Neil Sheehan, the agency’s spokesman.
“Based on plant owners’ responses to the bulletin and our requests for additional information, the NRC staff determined that appropriate interim corrective actions (fixes) have been implemented at all affected plants,” Sheehan wrote in an email. “As such, the risk associated with an (open-phase condition) is significantly reduced.”
At Pilgrim, its existing system was supplemented by interim safeguards that include twice daily checks by employees of the switchyard, where the plant’s lines connect to offsite power.
Entergy spokesman Patrick Thomas O’Brien said via email that “because of the way Pilgrim receives power, the station is unlikely to be affected by these potential concerns. In addition, Pilgrim has an existing automatic detection system that mitigates this concern, and given that the station will permanently shut down not later than mid-2019, additional upgrades are not planned.”
The NRC will follow its usual procedure for reviewing formal petitions when considering the petition from the engineers, Sheehan said.
The group has asked federal regulators for a response by March 21.
It was a brave move for a group of employees to file a petition, Lochbaum said, and also signals the seriousness of the problem.
“If employees of the NRC do not trust the NRC to have acted to protect members of the public and have to petition their employer to protect the public, why should any member of the public trust the NRC to have its back (other than to have its back covered with a target)?” Lochbaum wrote.